Thursday, 29 May 2014

The Domestic RHI and Solar Thermal Stores

The Law of Unintended Consequences Strikes Again

The domestic RHI was structured with the intent that the complementary combination of solar thermal with other heating technologies would be actively encouraged by receiving double subsidy for the domestic hot water energy.  Unfortunately, the wording of the legislation has prevented installers using the simplest way to implement a combined system (the thermal store) because it rules out solar systems that can make even a theoretical contribution to space heating.

Thermal Stores in Hot Water

Solar thermal systems can make a contribution to space heating as well as domestic hot water (DHW) preparation, especially in spring and autumn where the days are still bright and there is a demand for space heating.  These systems are not yet as common in the UK as those for domestic hot water, but in more developed European markets such as Germany and Austria, so-called "solar combi systems" are popular.

In a thermal store the domestic hot water is heated in a heat exchanger
and the contents of the store pumped around the space heating circuit

A good way to combine solar thermal with space heating is to use a thermal store, essentially a large (typically 500 litre minimum to 1,000 litre) hot water cylinder with heat inputs from both solar and the backup heating system and with outputs to domestic hot water and space heating.  Typically the body of water in the thermal store is heating system fluid (primary water) and domestic hot water is heated on-demand in a heat exchanger as it flows to the hot tap.

Both heat pumps and biomass heaters operate well when running continuously rather than cycling on and off, so charging a thermal store is a good technical solution that improves the overall efficiency of the heat pump or biomass boiler.

Where the designer is seeking for the solar to make a reasonable contribution to the space heating, the solar panel array installed is large (around 12-18 m2 for a domestic property).   The coverage of domestic hot water of such systems can be very high, 70% and above.

Where the designer is aiming for solar to mainly cover domestic hot water the panel array is smaller (typically in the range of 3  - 6 m2).  In this case there is still a theoretical possibility that the solar energy will contribute to the space heating, though in practice the system is sized with the aim of supplying 60-70% of water heating.
The current domestic RHI legislation completely excludes systems that can contribute towards space heating.  

The text in the RHI regulations defines an eligible solar system as follows:

a)     is designed and installed to provide heating solely to a single eligible property and solely for an eligible purpose using liquid as a medium for delivering that heat;

(b) meets the requirements set out in whichever of the standards for solar thermal plants specified in paragraph 1(5)(a) and (b)“eligible purpose” means, in relation to heat generated by— […](b) a solar thermal plant, the purpose of domestic hot water heating for an eligible property;

An implementation of solar where there is even a theoretical possibility of the solar contributing towards space heating is completely excluded from the scheme.

The reasoning behind ruling out solar space heating was that the domestic RHI is “deemed” – the solar energy is not measured, instead it is estimated using an approved calculation and the calculation only works for domestic hot water.

However, by ruling out any solar installation that does not solely heat domestic hot water, the domestic RHI has made the combination of complementary renewable heating technologies such as solar and heat pumps less likely. Solar thermal has lower associated carbon emissions than any form of back up heater, so every unit of solar thermal heat that can be used, whether for space heating or domestic hot water reduces carbon emissions.

Configurations where the solar is offsetting a proportion of fossil fuel space heating are also disincentivised by their complete exclusion from the domestic RHI.

When installing biomass or heat pumps with a thermal store, the additional cost to add a solar coil into the store is very low, making the marginal cost of adding solar thermal more attractive.  The domestic RHI would provide greater value for money if it encouraged, rather than discouraged such systems.

So how could the domestic RHI be changed to include solar space heating?

Two Suggestions

Two options occur, though I’d be pleased to hear of any other suggestions (please use the comments section).

First, it would clearly be possible to use a heat meter to measure the solar input into the thermal store.  Solar space heating systems cost more than solar systems aimed only at domestic hot water.  A requirement to fit a heat meter would be a relatively smaller proportion of the total installed cost and energy benefits, and houses that can fit large thermal stores are relatively thin on the ground, so it wouldn’t be too much of a cost for the scheme administrators to deal with the meter readings.

A second approach would be to allow space heating systems onto the scheme but to give RHI payments only for the domestic hot water energy provided, and calculate this with the current deeming method.  I’ve looked at this with the help of two years'  of data from a solar space heating system provided by Geoff Miller of GreenLincs Energy.  Simulations have also confirmed that the solar energy generated by a system providing solar space heating and domestic hot water is always higher than the same sized system targeted at only domestic hot water.  The RHI wouldn't be over-paying for solar heat.

The best outcome would be for it to be the choice of the homeowner whether or not to go to the expense and hassle of having a heat meter.  If they wanted the extra payments for space heating, then they would need to install a heat meter, otherwise they could claim for only the solar heat in their domestic hot water.

This has formed the basis of a proposal submitted to the Department of Energy and Climate Change (DECC) yesterday outlining how the scheme could be improved by allowing solar space heating.


  1. Stuart I have also been wondering about this issue - one of the points of the RHI should be to improve understanding of the impact of renewable heating technologies on heating the home - especially in the case of hybrid systems which will inevitably be more difficult to predict or model. Why else would an optional heat metering package be available for the owners of biomass or heat pump systems under the dRHI if collecting useful data was not the point?

    The logical conclusion is obviously to meter solar input - but I would go a step further and say that the homeowner shouldn’t have to pay for the heat meter, but should be given the extra £200-230 that is offered to biomass or heat pump owners, as you would be providing equally useful data.

    At present the payments made to BM/HP owners are still taken from the deemed amount rather than the metered amount in this situation. It would seem to me that this is fair given the extra payment - but doesn’t answer the question about how you then incentivise the “heating” input for solar thermal.

    I can think of a couple of possible solutions - one is to have a new tariff just for combi cylinder systems where metering is obligatory and payment is made on the measured amount, probably with a different p/kwH rate but that would take some thinking about. Another is to have a “deemed plus” type arrangement where you are given the deemed hot water calculation amount + an adjustment payment. This is potentially quite fraught because you don’t want to incentivise massive systems that offer diminishing returns and will stagnate frequently in the summer - so probably better to keep this simple and leave that as based on the size of the property / heat load from the EPC?

  2. Stuart, this is a very helpful explanation although the issue is very frustrating. I have just installed 3 x 3sqm of Viridian roof integrated panels with a 300 litre Xcel Heatbank thermal store so will be expecting the solar thermal to contribute to our space heating in the spring and autumn. We are using a plate heat exchanger instead of a solar coil but don't seem to have it set up right yet.

    1. Tim

      yes, it's really poor that people installing solar in this way can't access the government support. I'm not sure what will happen if we do get the rules changed - whether 'legacy' installations will suddenly become eligible in retrospect. If I had to bet money, I'd have to guess not.

      PS- please email me a nice photo of your house with the panels - bottle of wine if we use it in our gallery!


  3. I didn't appreciate the nonsensical rule on thermal stores and now it looks like I will lose my RHI payments. Really angry about this it makes no logical sense whatsoever.